The present invention relates to a time-axis correcting circuit for a recorded data reproducing device.
In a recorded data reproducing device, the reproduced signal can suffer from a time-axis error component attributed to nonuniform operation of the drive section. In a video signal reproducing device such as a video disc player or a video tape recorder, the reproduced picture is considerably adversely affected by such a time-axis error component included in the reproduced signal. Therefore, in order to improve the quality of the reproduced signal, the time-axis error component should be suppressed as much as possible.
In order to perform time-axis correction of the reproduced signal, a digital TBC device has been proposed which converts a reproduced signal into a PCM signal which is stored in a memory to thus absorb the time-axis error component. However, the use of a digital TBC device is not suitable for ordinary consumer use because the device is large in size and expensive. A time-axis correcting circuit proposed for consumer use is shown in FIG. 1 in block diagram form. In FIG. 1, reference numeral 1 designates a variable delay circuit capable of changing the delay time between the input signal and the reproduced output signal according to a control signal; 2, a separating circuit for separating timing data from the reproduced signal; 3, a reference signal generating circuit for producing a reference signal having a frequency equal to that of the timing data when no time-axis error component is present; 4, a phase comparator circuit for comparing the timing data separated by the separating circuit 2 with the reference signal to detect a time-axis error component of the timing data; and 5, a phase correcting circuit for determining the characteristics and stability of the closed-loop control system and which provides the control signal applied to the variable delay circuit 1.
In the phase comparison circuit 4, the reference signal and the timing data are compared in phase to extract the time-axis error component of the timing data as a time-axis error voltage. The latter signal is applied to the phase correcting circuit 5. The phase of the reproduced signal output applied to the variable delay circuit 1 is controlled according to the control signal provided by the phase correcting circuit 5. Accordingly, when the reproduced signal including the time-axis error component passes through the variable delay circuit 1, the time-axis error component is suppressed so that the reproduced signal is subjected to time-axis correction.
However, in the conventional time-axis correcting circuit, it is difficult to obtain precise coincidence of the frequency of the reference signal and the frequency of the timing data when no time-axis error component is present. Accordingly, the time-axis error voltage produced by the phase comparison circuit 4 includes a DC component corresponding to the frequency difference. The DC component of the time-axis error voltage may exceed the dynamic range of the closed-loop control system, and therefore the closed-loop control system may become unstable.
In order to overcome this difficulty, a time-axis correcting circuit as shown in FIG. 2 has been proposed. In FIG. 2, those components which have been described with reference to FIG. 1 are designated by the same reference numerals and characters. Only components which are different from those in FIG. 1 will be described.
In FIG. 2, reference numeral 6 designates a flywheel oscillator provided for detecting the time-axis error component of the timing data. The flywheel oscillator 6 is a PLL (phase-locked loop) circuit, including a voltage-controlled oscillator circuit. The closed-loop control characteristic of the PLL circuit is determined to lock the phase of the output signal to the low frequency time varying component of the phase of the timing data. The voltage-controlled oscillator circuit is coupled to a phase comparator inside the PLL circuit. The phase comparator detects the amount of phase shift of the timing data from the phase of the reference signal outputted by the voltage-controlled oscillator circuit and applies the resulting detection signal to the phase correcting circuit 5. The flywheel oscillator 6 itself is a stable, closed-loop control system, having a frequency characteristic including no intrinsic servo loop oscillation frequency.
In the time-axis correcting circuit utilizing the flywheel oscillator 6 constructed as described above, the frequency of the reference signal coincides with the frequency of the timing data when no time-axis error component is present, and the above-mentioned drawbacks accompanying the conventional time-axis correcting circuit of FIG. 1 are therefore eliminated. However, in a video disc player, the speed of the disc is set to typically 1800 r.p.m., and the main component of the frequency variation caused by the eccentricity of the disc is limited to 30 Hz. Thus, a recorded data reproducing device produces a separate, dominant time-axis error component. In the conventional time-axis correcting circuit of FIG. 2, the flywheel oscillator 6 can only detect a time-axis error component which is a high frequency time varying component of the phase of the timing data. Therefore, the conventional time-axis correcting circuit suffers from a difficulty in that it can suppress time-axis error components other than the dominant time-axis error component, but it cannot suppress the dominant time-axis error component.
In order to overcome this difficulty, the present applicant has proposed a time-axis correcting circuit which can suppress such a dominant time-axis error component without significantly increasing the circuit size. This circuit is described in Japanese Patent Application No. 215606/1982 and U.S. patent application Ser. No. 560,097, filed Dec. 12, 1983, which are described here to provide a better understanding of the present invention. This time-axis correcting circuit will be described with reference to FIG. 3. In FIG. 3, reference numeral 7 designates an input terminal for a reproduced signal; 8, an output terminal for a reproduced signal; 9, a variable delay circuit; and 10, a separating circuit employed as a timing data extracting circuit. A video signal is applied, as the reproduced signal, to the input terminal 7. The color burst signal of the video signal is utilized as the timing data. The variable delay circuit, which may be a charge-coupled element such as a CCD (charge-coupled device), operates to vary the delay time of the video signal. The separating circuit 10, implemented, for instance, with a 3.58 MHz B.P.F. burst gate circuit, separates the color burst signal from the video signal. Further in FIG. 3, reference numeral 11 designates a flywheel oscillator including a voltage-controlled oscillator circuit 11a, a phase comparator 11b, a sample-and-hold circuit 11c, and a phase correcting circuit 11d. The voltage-controlled oscillator circuit 11a produces a reference signal used for time-axis error detection. The phase comparison circuit 11b compares the phase of a color burst signal and the reference signal and produces a signal representing that difference. The sample-and-hold circuit 11c latches the time-axis error of the color burst signal, which is present discontinuously for a period of time corresponding to the horizontal synchronizing interval, to obtain a continuous time-axis error signal. The voltage-controlled oscillator circuit 11a, the phase comparison circuit 11b, the sampel-and-hold circuit 11 c and the phase correcting circuit 11 form a sub servo loop.
Further in FIG. 3, reference numeral 12 designates a phase correcting circuit, and 13, a voltage-controlled oscillator circuit utilized as a drive circuit. The phase correcting circuit 12, the voltage-controlled oscillator circuit 13, the variable delay circuit 9, the separating circuit 10 and the flywheel oscillator 11 form a main servo loop. The phase correcting circuit 12 determines the control characteristics and stability of the main servo loop. The variable delay circuit 9 controls the delay time of the output video signal according to the clock output from the voltage-controlled oscillator 13, the frequency of which varies in response to the voltage of the output of the phase correcting circuit 12.
In sub servo loop of the flywheel oscillator 11 has a characteristic that the phase of the reference signal is locked to the low frequency time varying component of the phase of the color burst signal and the frequency of the reference signal is made the same as the frequency of the color burst signal when no time-axis error component is present. The sub servo loop itself has an oscillating characteristic (is stable). The oscillation frequency thereof is set to a value equal to the frequency of the dominant time-axis error component of the video signal. When the sub servo loop operates forming a part of the main servo loop, it is stable and does not oscillate.
In the above-described conventional time-axis correcting circuit, the dominant time-axis error component together with other time axis-error components is suppressed, and therefore the reproduced signal is improved in quality without increasing the size or complexity of the circuit significantly. However, since the time-axis error components other than the dominant time-axis error component are suppressed, for instance, by adjusting the characteristics of the phase correcting circuit 12, they cannot be totally suppressed. Especially in the case where the dominant time-axis error component is not of a single frequency, the suppression of the dominant time-axis error component is inadequate.
For instance, in a video disc player for reproducing a CAV disc rotating at a constant speed, the dominant time-axis error component is limited to a single frequency of 30 Hz. On the other hand, in a video disc player for reproducing a CLV disc rotating at a constant speed, the speed is 1800 r.p.m. at the innermost periphery and 600 r.p.m. at the outermost periphery, and hence the frequency of the dominant time-axis error component is distributed over a range of from 10 Hz to 30 Hz. Therefore, in the case where the dominant time-axis error component is distributed as described above, suppression cannot be accomplished sufficiently with the conventional time-axis correcting circuit in which suppression is carried out with a single frequency taken into account.
In view of the above-described difficulties accompanying a conventional time-axis correcting circuit, an object of the invention is to provide a time-axis correcting circuit for a recorded data reproducing device in which, even when the dominant time-axis error component is not of a single frequency, suppression can be sufficiently achieved without significantly increasing the circuit size or complexity.